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Fig. 6 | Zoological Letters

Fig. 6

From: The phylum Vertebrata: a case for zoological recognition

Fig. 6

Head–trunk interface. a to c. Schematic representation of the head and trunk in the pharyngula of modern jawed vertebrates, as defined by the migratory/distribution patterns of neural crest cells (NCCs). In the head of the vertebrate pharyngula, NCCs form extensive ectomesenchyme with three major NCC populations, called trigeminal (tc), hyoid (hyc) and circumpharyngeal crest cells (cp), filling the frontonasal region and pharyngeal arches, defining the vertebrate head (yellowish region in C), as opposed to the posterior domain occupied by somites and the lateral plate. In the trunk, NCCs are segmented primarily into somitomeric streams by the presence of somites (dark green). Between the two distinct groups of NCC populations is found an S-shaped interface (red broken line). Circles denote placodes for cranial nerves (oph, ophthalmic placode; mm, maxillomandibular placode; gn, geniculate placode; pet, petrosal placode; nd, nodose placodes). In B, the position of trapezius muscle development (tr) and pathway of the hypobranchial muscle (the hypoglossal cord: hyp) are shown along the head–trunk interface. d. Schematization of an early lamprey larva. Note that the head and trunk can be defined in this animal as a vertebrate-specific feature. e. Comparison with schematized amphioxus. The mesoderm of this animal is entirely segmented into somite-like structures, but there is no overt lateral plate. Because the pharynx is located medial, not ventral, to the body wall, the head–trunk interface cannot be defined in this animal. mn, mandibular arch; mo, mouth; ot, otic vesicle; p, pharyngeal pores in amphioxus; p1 to 8, pharyngeal pouches; PA2 to 4, pharyngeal arches; s, somites

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